Microwave attenuation units



Jan. 17, 1961 MICROWAVE ATTENUATION UNITS Filed Oct. 22, 1956 2Sheets-Sheet l EUL 6 25 24 40b 40 40c 42 4/ 20 J? Z7 Z8 M /7 r m i WM WT E ,4; 32 AT -;7 g I r 60 3 M 1 X/A/ I 1 ,z/ :1 3.; 2a

4'4 Arman H Jan. 17, 1961 MICROWAVE ATTENUATION Filed Oct. 22, 1956 ix?//Y c/oz/vr /6' W M. J. RODRIGUEZ UNITS 2 Sheets-Sheet 2 BY F MQMMICROWAVE ATTENUATION umrs Michael J. Rodriguez, Bayside, N.Y., assignorto Empire gevlices, Inc., Bayside, N.Y., a corporation of New Filed Oct.22, 1956, Ser. No. 617,551

12 Claims. (Cl. 333--81) This invention relates to microwave attenuationunits, more particularly to attenuators operative from direct current tofrequencies in the order of 10 kilomegacycles and higher, with lowvoltage standing wave ratios.

It is a general object of the present invention to provide novelattenuation units usable in microwave circuitry; of general constructionrelated to coaxial lines and cables, and readily insertable in suchcables; and operative throughout the frequency range from direct currentto 10 kmc. signals and above with low reflection and with predeterminedinsertion or attenuation loss. Such attenuation units are constructedwith specific impedance characteristic, and with physical dimensions tocoact with the coaxial line or cable with which they are used.

In the practical design or operation of circuits or components atrelatively low frequencies, electro-magnetic field considerations andwave propagation are usually neglected. The physical size of circuitcomponents at low frequencies is small compared to the relative wavelength of such frequencies. Thus, the effects of voltage and current maybe considered as established instantaneously throughout the circuit,when transient effects are of no concern. Established engineeringconcepts of voltage and current utilizing Ohms law and otherformulations of circuit interactions are practical for relatively lowfrequencies, without the aspects and mathematics of complexelectromagnetic field theories.

However, as the frequency of the electromagnetic energy utilized in thecircuit is increased to the microwave region, above 1 kmc. for example,the physical size of the circuit components and elements is no longernegligible with respect to the wave length. At frequencies above 1kilornegacycle it is impractical to neglect the field conditions of theelectromagnetic energy, nor is it practical to consider the circuitcomponents or elements as lumped as is done in lower frequency analysisand circuitry. In the microwave field, the line integral of the electricfield strength taken between two points at one instant of time, is nolonger independent of the path. The same applies to the current. Theusual low frequency concepts of current and voltage do not apply in themicrowave region, in fact, the ordinary concepts of capacitance andinductance tend to lose their identity.

In accordance with the present invention, novel means are utilized totransmit microwave signals from a coaxial line or cable through aconical resistive section which uniformly attenuates the signals withnegligible reflections up to frequencies in the order of 10kilomegacycles and higher. The conical surface has a film resistancewhich film is thinner than the smallest depth of penetration,corresponding to the highest operating frequency. It may be linear orexponential in longitudinal cross-section, as will be set forth in moredetail hereinafter. The essential improvement provided by the conicalresistor elements of the invention, in the path of coaxially fedmicrowave signals, is that they effect negligible disturbance of theelectrical and magnetic fields fates Patent F 2,968,774 @atented Jan.17, 1961 constituting the microwave signals in their passage across theconical resistive elements.

The passage of microwave signals along the conical resistive surfaces ofthe invention attenuators, results in gradual attenuation of the signalsin a predetermined manner, and allows the fields thereof to vary withvery little distortion. The conical structures of the present inventioninhibit reflections of the electric and magnetic fields of the microwavesignals, and result in a minimum practical standing wave ratio. Furtherin accordance with the present invention, two conical resistive unitsare arranged back-to-back and utilized as symmetrical or twowaytransmission attenuation units, such as T-pads.

In its practical aspects, the present invention simplifies theconstruction, testing requirements and basic cost in the fabrication ofattenuation units such as T-pads, pi-pads, terminations, and the like.The conical resistive elements of the present invention are thin filmsdeposited upon conical dielectric structures of good low-loss dielectricmaterial, such as glass, steatite, etc. The conical elements are moldedto final shape, or cured and machined if required. The film ofpredetermined characteristic and resistance, as will be set forth, isthereupon applied thereto.

Such conical resistor arrangement, whether linear or exponential inlongitudinal cross-section, provides the advantageous transmission anduniform attenuation action upon signals from DC. up to the 10 to 12Kare. range, without requiring corresponding tapering of the cylindricalshield or casing of the attenuation unit. For such applications, it isless expensive to provide conical tapering of the central resistors; andthus provide the very high-frequencies uniform passage, with negligibleVSWR, than by corresponding matching by close machining of thecylindrical casing or shield. However, by providing tapering in thecylindrical shield about the invention conical resistor, a second orderof close matching is afforded to further enhance the ability of theattenuation device to perform its designed function with lower VSWR, aswill be set forth in detail hereinafter.

It is accordingly an object of the present invention to provide novelattenuation units operative from direct current to the 10 to 12 kmc.microwave range, uniformly, and with minimum VSWR.

Another object of the present invention is to provide coaxialattenuation units embodying novel conical resistor elements for use onsignals to the order of 10 kmc., and with uniformly low VSWR.

A further object of the present invention is to provide novel coaxialattenuator pads embodying conical resistive elements rendering such padsrelatively inexpensive, andfor extended high frequency application inthe microwave range with low VSWR.

Still another object of the present invention is to provide novelcoaxial attenuator units embodying conical resistive elements, includinglow-loss dielectric material based structure and adapted for design andconstruction in various forms, such as T-pads, pi-pads, L-pads,terminations, etc.

Still a further object of the present invention is to provide a novelT-type coaxial attenuator pad, operable from direct current up throughthe 10 to 12 kmc. microwave range, with low VSWR throughout the range,from either direction, and for specific impedance and attenuationcharacteristics.

These and further objects of the present invention will become moreapparent from the following description of specific embodiments thereof,taken in connection with the drawings, in which:

Fig. 1 is a cross-sectional view of the coaxial T-pad constructed inaccordance with the present invention, for

use from direct current through the 10 kmc. range, and above.

Fig. 2 is a perspective view ,of the central double-conical resistiveelement utilized in the T-pad of Fig. 1.

Fig. 3 is an elevational view of a modified form of double-conical unit,of the exponential type.

Fig. 4 is a cross-sectional view of the basic portion of a modifiedbidirectional T-pad attenuator, utilizing the double-conical exponentialresistor of Fig. 3.

Fig. 5 is a further form of attenuation unit, being an L-pad utilizingthe double-conical element of Fig. 2.

Figs. 6 and 7 are alternate forms of the series resistor elements forthe attenuation units.

Fig. 8 is a cross-sectional view through the central section of a pi-padcoaxial attenuator, utilizing conical resistive elements in accordancewith the present invention.

Fig. 9 is a cross-sectional view through a termination unit utilizing aconical resistor element.

Fig. 10 is a schematic diagram for the conical resistor surface angleanalysis.

Attempts have been made to construct coaxial attenuation unitsincorporating lossy dielectric material. However, it is known to thoseskilled in the art, that while such units encompass microwavefrequencies, they are selective over particular band-widths. Theembodiment of lossy dielectrics in an attenuation unit restricts itsoperation at low frequencies and direct current, and does not lenditself to uniform VSWR characteristics.

The attenuation units of the present invention embody thin resistivelayers, coatings or films on cores of good or low-loss dielectricmaterial, such as steatite, zirconium, glass, quartz, and the like.Bushings, contacts, sleeves, etc. of the attenuation units are made of agood conducting metal, with smooth or closely machined surfaces, andwithout burrs or other irregularities that would introduce distortionsin passing microwave signals. Fig. 1 is a crosssectional view of anexemplary embodiment of a T coaxial attenuation pad constructed inaccordance with the present invention. The coaxial attenuator pad 20 isarranged with the usual terminal hardware 21 and 22 for coaxial lines orcables to which the attenuator unit 20 is inserted in series. It is tobe understood that the characteristic impedance design of attenuator 20is made equal to that of the characteristic impedance of the associatedcable or line circuit. The insertion loss or attenuation of attenuatorunit 20 is predetermined, and for example may be 10 db, 20 db, or anyother desired value to which the unit lends itself constructionwise.

Attenuator pad 20 comprises outer cylindrical metallic support andshielding member 23. Coaxial fitting 21 is secured to the left end ofsleeve 23 through set screw 24, engaging collar 25 of member 21. Arotatable knurled sleeve 26, having internal threading, is for engaginga cooperative section of the coaxial line or cable to which pad 20 iscircuitally attached. Similarly, the right end of pad 20 has threadedhardware member 22 secured to sleeve 23 by set-screw 27 engaging collar28 of member 22. The connection members 21 and 22 have centralconducting stems which engage the central elements of the T-pad throughrespective conductive rods 31, 32 supported within collars 25, 28 byrespective dielectric bead supports 33, 34. The outer sections ofmembers of 21, 22 are of metal and conductive through their respectivecollar 25, 28, and are contiguous electrically with conductive shield 23of pad 20. Such outer conductive members constitute the returnelectrical path for the microwave signals, and provide continuousshielding for the cable connections and pad 20. As the connectionhardware members 21, 22 do not constitute part of the present invention,further details thereof are not shown.

Centrally of T-pad 20 is a double-conical resistor element 35.Bi-conical resistor 35 has a central annular ridge 36 extending from thebasic juncture of the two cones 37, 38 extending toward the shieldingsleeve 20. Central ridge 36 is conductive. The annular ring 36 is lockedinto position between opposing internally tapered cylinders 40 and 41.The juncture of annular ridge 36 on either side with the contiguoussurfaces of cylinders 40, 41 is made firm, and closely proportioned foraccurate positioning and rigid maintenance of contact of the cylinders40, 41 and the cones 37, 38 of bi-conical resistor 35, for reasons to bemore fully set forth hereinafter. A thin conductive metallic shim 39 ofresilient material is interposed between the inner surface of outercylinder 23 and the cylinders 40 and 41, as well as the outer surface ofthe annular conducting ring 36 of unit 35. The purpose of resilient shim39 is to coordinate the electrical and mechanical factors of pad 20,whereby firm electrical and mechanical interconnections are made betweenunits 40, 41 and ring 36 with the surrounding shielding cylinder 23throughout the associated circumferential areas thereof.

Fig. 2 is a perspective illustration of bi-conical resistor 35. It is tobe noted that the hollow interior 42 (Fig. l) of resistor 35 iscylindrical and concentric with outer sleeve 23. The internal surface 43of the hollow cylinder 42 with bi-conical resistor 35 is preferablyconductively plated, such as with silver, to establish a uniformconduction path through the interior of the resistor 35. Also, it is tobe noted that centrally through hollow cylinder 42 within resistor 35 isa brass rod or screw 45 for mechanically securing conducting cups orclips 46, 47 to the end openings 48, 49 of the bi-conical resistor 35.The open end circular portions 48, 49 of bi-conical resistor 35 eachcontain an annular area which is silvered or otherwise made conducting,for establishing, with the sliver surfaced interior 43 firm electricalconnection with the respective contacting clips 46 and 47. The conduction path between resistor end-clips 46 and 47 is basically provided bythe conductive cylinder surface 43 in central region 42 of unit 35.

Colinear with connector clips 46, 47 and extending outwardly therefromare series resistors 50, 51 respectively. Resistors 50, 51 are madepreferably with similar ceramic core material as bi-conical resistor 35,such as of steatite, or zirconium, etc. and are faced with a suitablefilm of resistance material such as of carbon, boron, etc. Suchresistance material may be deposited, diffused, plated or otherwisesuitably secured to the ceramic core, in a manner well known in the art.

Fig. 6 is a perspective illustration of series resistor 50, which isidentical to resistor 51. Resistor 50 has a thin ceramic core 52 whichis otherwise hollow, but may be a solid cylinder if desired. The ends53, 54 of ceramic cylinder 52 are conductive annular strips such as ofsilver, for connection to corresponding connector clips. Con ductiveannular areas 53, 54 are preferably formed after the resistor film 55 isdeposited upon the ceramic base 52, to insure continuity of connectionacross the series resistor 50.

Annular conductive section 54 of series resistor 50 nests withinconnector clip 46 in Fig. 1. The opposite end annular connection area 53nests within connector clip 56, that extends from central conductor rod31 of coaxial hardware member 21. Similarly, the.left end of seriesresistor 51 is in firm electrical contact with connector clip 47; andits right end, with connector clip 57 extending from central conductorrod 32 of coaxial member 22. The use of the connector clips and thesilvered annular areas on the resistors 50, 51, as well as on bi-conicalunit 35, is to afiord firm electrical interconnection and support of theparts of the T-pad, and to permit practical and economicalmass-production of the elements separately and effective assembly of thepad in its manufacture. These respective elements and components, whenassembled, constitute a unitary body due to their alignment, theirfitting, and their respective dimensions, for all practical applicationsof the pad.

It is important that the relative dimensions of the components, andmechanical stability of the unit be maintainfid i service, as theuniform attenuation characteristics of T-pad 20 up through It) to 12kmc. microwave signals is dependent upon the parameters of design andthe relative tolerances of the parts, as will be understood by thoseskilled in the art. Towards this end, the annular ridge '36 is a veryimportant factor in maintaining a basic anchor for the integral conicalresistors 37 and 38 of member 35, and also through the extendingconnector clips 46, 47 maintaining series resistors 50, 51 in theirconcentric position in pad 2%. The initial coaxial align ment andmaintenance of conical resistors 37, 38 within pad Ztl is assured byproper fabrication of annular ridge 36, and its coaction with thetapered cylinders 40, 41 as heretofore described.

It is well established in the microwave art that the characteristicimpedance Z of a section of a coaxial structure is expressed as follows:

where Z, is the characteristic impedance along a plane in the line, theouter diameter is D, d is the (inner) diameter of the elements in thecenter; and e is the dielectric constant of the material therewithinalong which the signals pass. For air e equals 1. Maintenance of thecharacteristic impedance close to a predetermined value, or changing itgradually where necessary, minimizes or maintains the VSWR figure lowfor the frequency range. Towards this end, I will now detail the novelfeatures and factors of T-pad 20 in accordance with the presentinvention.

Starting at the left end, we have connection clip 56, which isconductive and of uniform diameter across its axial extent. Accordingly,the corresponding portion 40 of tapered cylinder at) is made flat,namely of uniform axial diameter, to correspond with the uniformdiameter of clip 56. The characteristic impedance Z which is determinedby the diameters in the above formula, is made equal to the impedance ofthe system for which pad 20 is designed. We next come to linear seriesresistor 50, having a uniform resistive film deposition across its outersurface in the axial direction of pad 20.

It is established in the microwave art that for a coaxial pad embodyinga linear resistance and a uniform resistive change R per unit of length,that its characteristic impedance is Z=R-dx. The result, as isestablished in the art, and shown for example in US. Patent No.2,399,645, is to use a logarithmic cone as the shielding cylinder aboutthe linear resistor 50, to maintain a mathematically perfect transitionor uniform characteristic impedance (z). However, a linear taper 40subtends linear resistor 50, to provide a practical smoothcharacteristic impedance transition throughout the microwave signal pathacross linear resistor 50. It is more expensive to provide a truelogarithmic shape for surface 40 of cylinder 40. For frequencies from DCto 12 kmc., I have found that a linear taper for surface 40 of cylinder40 maintains the VSWR well within 1.2 throughout the range. Should ahigher frequency range be desired, the linear cylindrical resistor 50may be replaced with the tapered resistor 6i} described hereinafter inconnection with .Fig. 7. i

The next section in the T-pad 20 is connection clip 46 between resistors59 and 37. Since the charracteristic impedance across the axial path ofciip 4-6 is constant, a flat section 46 is built into cylinder 40 tokeep the characteristic impedance at that section of pad 20 constant andequal to the impedance appearing at the right end of resistor 59, and isdetermined by the contiguous smallest diameter in the taper of surface40*.

We next arrive at the left terminus of conical resistor 37 of bi-conicalunit 35. It is to be noted that resistor 37 progresses conically byincreasing diameter in the planes perpendicular to the path of themicrowave signal, from left to right herein, namely from the diameter ofclip 46 to the portion of the annular connection ring 36 where it meetscylinder 40. The conical surface 58 of resistor section 37 has depositedfirmly thereon a resistive film such as of carbon, boron, or the likewell known in the art, and of proper resistive value to serve as theshunt element in the T-pad 20. The transitional characteristic impedancechange from the position at clip 46 of conical resistor 37 to theannular connection ring 36 thereof is linear in effect, resistively, andalso physically linear by its tapering.

The tapering configuration of conical shunt resistor 37 affords a firstorder logarithmic compensation effect for the characteristic impedancealong the axial direction of pad 20 thereat, which per se has been foundto eifect a smooth transition in the characteristic impedance commensurate with the linear resistance change. In other words, if theopposing surface 40 of cylinder 40 were untapered, namely flat, the veryfactor of tapering the surface 58 of conical resistor 37 compensates forthe linear resistance change and produces a smooth transition of thecharacteristic resistance from one end of conical resistor 37 to theother. It is inherently compensating for all practical purposes, andgives a first order compensation or correction corresponding to theformulation otherwise required by more complex construction, and iseffective in all the practical attenuation units hereindescribed. I havefound by experimental determination on a unit as per T-pad 20, thatuniform passage and attenuation of microwave frequencies to 10 to 12kmc. and extending through all frequencies down to direct current, areattenuated uniformly and with VSWR below 1.2 in all cases.

For practical purposes, the concentric portion of cylinder 40 aboutconical resistor '37, namely surface 40 is tapered by a straight taperfrom the smaller diameter at 4% to the maximum diameter at the resistiveportion of cone 37 at sleeve portion 40 where it is contiguous withconductive annular ring 36 at the central hub of the bi-conical resistor'35. The practical tapering of section 40 extends thereby a furthercorrection effect about the otherwise tapered disc resistor 37,producing a second order of correction and closer to the theoretical. Inpractice, I have found that such arrangement is sufficient to provideuniform attenuation with uniformly low VSWR for the frequencies fromdirect current up through 12 kmc., and even higher.

It is to be noted that the right-hand half of attenuator pad 2t) isidentical in construction, electrically and mechanically, with thesection left of the annular ring 36, and in effect, is a mirrorsymmetry. The theory and practical operation of the right-hand sectionembodying conical resistor 38, connection clip 47, series linearresister 51, and clip 57, are otherwise identical to that hereinabovedescribed for the left half of attenuation unit 20. Transition of thesignals from the central sector at the annular connection ring 36, tothe right end 32 of attenuation pad Ztl will now be understood by thoseskilled in the art. Pad 2ft is symmetrical for signals from eitherdirection.

The pad 2 3 is designed to handle direct current as well as the uppermicrowave frequencies referred to. It is readily seen that theimpedances of elements 50, 51 and 35 are predetermined, and maintaintheir resistive values for the whole signal range to 10 to 12 kmc., withconstant attenuation, minimum reflection or distortion, and maintenanceof low VSWR. For example, in a 50 ohm characteristic impedance T-pad(20) for 20 db attenuation, series resistors 56 and 51 are slightly over40 ohms, and each of the conical sections 37 and 38 are about 20 ohms.Thus the characteristic impedance and the amount of attenuation for aunit of the invention is prescribed by predesign as set forth above.

Fig. 3 is a modified form of the bi-conical resistor 35 of Fig. 2. Thebi-conical resistor 65 of Fig. 3 has a central annular connection ring66 as in unit 35. However,

its individual conical resistor sections 67 and 68 are formed onlogarithmic shaped conical surfaces 69 and 70 respectively. Theelevational view Fig. 3 of unit 65 shows the logarithmically taperedcontour of the surfaces 69, 70 established between the respectiveconducting termini or annular ends 71 and 72 and the central annularring 66. A hollow cylindrical core 73 extends centrally through unit 65between conductive ends 71, 72, and is coated with a silver layer whichalso interconnects rings 71, 72. Signal conduction between rings 71 and72 is thus afforded.

Utilization of the logarithmically shaped conical resistors 67 and 68,constituting biconical unit 65, affords an additional order ofcorrection for the characteristic impedance change, in a practicalattenuation unit, per the formula for (Z) above. In other words,logarithmic bi-conical resistor unit 65, or separated half-sectionsthereof corresponding to 67 and 68 of unit 65, extends the upperfrequency range to which an attenuation unit will have uniformperformance, namely to 12 kmc. and above in the present examples. Also,for a given range, such as up to 12 kmc., the logarithmic unit 65 willperform with somewhat lower VSWR ratios than linear conical unit 35.

Fig. 4 is a cross-sectional illustration of the basic portion of anattenuator T-pad embodying the logarithmically shaped bi-conicalresistor 65 of Fig. 3. T-pad attenuator 75 of Fig. 4 corresponds inevery way to T-pad 20 of Fig. l, with the exception that logarithmicbi-conical unit 65 is used in place of linear bi-conical unit 35. Theouter sleeve and the connector fixtures of pad 75 are not shown, but areunderstood to be the same as those for T-pad 20. The series resistors 76and 77 are connected to the opposite end of bi-conical unit 65,contiguous with the respective resistor sections 67 and 68 throughassociated connection clips 78 and 79.

The tapered cylinders 80 and 81 are arranged about the respective leftand right halves of T-pad 75, in the manner of cylinders 40 and 41 ofunit 20. The internal surfaces 80*, 88 80 and 80 of cylinder 80correspond identically to the similar sections of cylinder 40 of T-pad20. The only differing feature of T-pad 75 from that of pad 20 is thelogarithmic configuration of the surface 69 of conical resistor 67, andof surface 70 of conical resistor 68. It is to be noted that section 88about the logarithmic surface 69 of conical resistor 67 is a lineartaper. As seen in Fig. 4, a corresponding condition prevails withrespect to conical resistor 68, and the whole right-hand half of T-pad75.

The result of utilizing logarithmic shaped surfaces 69 and 70 for theshunt conical resistors 66, 67 is to introduce a higher order ofcorrection for the characteristic impedance (Z) referred to hereinabove,in the T-pad. The electric and magnetic fields of microwave signalspassing through attenuator pad 75 maintain proper characteristicimpedance to minimize reflections and introduce the requisiteattenuation from direct current up through the top range of 12 kmc., andhigher, with low VSWR.

The pad 75 construction, with logarithmically corrected surfaces 69, 70for shunt resistances 67, 68, together with tapers surrounding theselogarithmic surfaces, corrects to frequencies even up to 15 kmc., withuniform attenuation, and with VSWR values below 1.2. The configurationof attenuation pad 75 affords a first order correction for the serieslinear resistor elements 76, 77 by the associated linear taperscorresponding to 80* of unit 80; and a third order correction for theshunt resistors 67, 68 due to the combined logarithmic and conicalconfigurations 69 and 7 of resistors 67, 68 as well as in thecooperating sleeve linear taper corresponding to 80 Fig. illustrates anL-attenuation pad. The L-pad 85 is illustrated in cross-sectional viewincorporating a basic bi-conical linear tapered resistive unit 35,corresponding to that shown in Fig. 2. The L-pad is illustrated with theouter cylindrical sleeve and cable terminal hardware elements omitted,but understood to correspond to those of Fig. l. L-pad is essentially aunit having a series resistor 86 corresponding to linear cylindricalresistor 55 of Fig. 6, and a bi-conical shunt resistor 35 in series withone end of resistor 86; and a conductive connection 87 back to thecentral connector of the coaxial cable hardware.

A metal cylindrical shield 87 is arranged about series resistor 86 andshunt resistor 37. Cylinder 87 comprises internal configurations whichcontain a series of steps 87' subtending linear series resistor 86; anda linear taper 87 subtending conical shunt resistor 37 For manyapplications herein a stepped configuration about linear resistors as 86has been found suitable in place of a logarithmic taper or of a lineartaper as (40 of unit 20). However, in place of the stepped section 87 ofcylinder 87, a linear taper or logarithmic taper may be used. Theutilization of linear taper 87 about conical resistor 37 provides asmooth gradual characteristic impedance change across this shuntresistive section of pad 85, for minimizing reflections and keeping theVSWR low, in the manner heretofore described for pads 20 and 75.

The section of pad 85 to the right of annular connection ring 36 andembracing conical resistor 38, is a simple cylinder 88 subtendingresistor 38 and connection clip 87. I have found that for many practicalapplications the first order of correction afforded by a conicalresistor section as 38 is sufficient for most applications even up tothe 10 kmc. range, with minimum reflections and low VSWR. It is feasibleto taper the internal section of cylinder 88 in the manner of taperedsection 48 of Fig. l, for further extensions of the frequency range forunit 85, or reduction of its VSWR. Fig. 5 is an illustrative embodimentof the application of a bi-conical resistor in an L-pad.

An alternate form for the series linear resistors is usable in thevarious attenuation units described herein. The linear cylindricalresistor 50 of Fig. 6 has been described hereinabove. The conical linearresistor 60 of Fig. 7 is similar to resistor 50, with the exception ofthe conical configuration of its body. Resistor 68 has a resistive film61 deposited thereon between the terminal annular conduction surfaces62, 63. When, for example, a conical series resistor such as 68 isutilized in place of series resistors 50, 51 in T-pad 29, or in place ofseries resistor 86 in the L-pad of Fig. 5, a higher order ofcompensation is realized, for the signals passing through the associatedsections of the attenuation units, resulting in an extension of themicrowave range to higher regions for a given VSWR, or lowering of theVSWR across that section, as will now be understood by those skilled inthe art.

Figs. 8 and 9 are illustrative exemplary applications of attenuationunits utilizing single conical resistors rather than bi-conicalresistors. Fig. 8 is a pi-attenuator pad showing the essential elementsthereof in cross-section, with the outer sleeve and hardware membersomitted. The pi-pad 98 comprises essentially two conical shunt resistors91 and 92, connected to central conductors 93, 94 respectively of thecoaxial system, and containing a series linear resistance 95 in seriestherebetween. Series resistor 95 is supported at its left end at conicalresistor 91 through connection clip 96. Connection clips 93 and 96 areelectrically interconnected through central conduction layer 183. Theright end of resistor 95 is electrically and mechanically connected withconnection clip 97 extending from resistor 92. Conduction layer 184centrally through resistor 92 interconnects clips 94 and 97. The conicalresistors 91, 92 contain annular support and connection rings 98 and 99respectively, corresponding to ring 36 of bi-conical element 35. Annularrings 98 and 99 are plated with conducting material and electricallyconnect the corresponding ends of conical resistors 91, 92 to outercylindrical shield members 100, 101 and 102 respectively.

Cylindrical members 100, 101, and 102 are in turn connected to an outerpi-pad cylinder (not shown, but corresponding to cylinder 23 of Fig. 1).The securement of annular rings 98 and 99 between cylinders 100, 101 and102 are for firm electrical connection and for accurate positioning ofthe conical resistors 91, 92 in the pad 90 assembly. The conicalrmistors 91, 92 through their central bores 103, 104 support theconnection clips 93, 94, 96, 97, and in turn linear resistor 95, tocomplete the pi-pad circuit. A tie-rod or screw may be used tointerconnect clips 93-96 and 9497 mechanically, as rod 45 of Fig. 1. Itis to be noted that there is no tapering of outer cylinders 100, 101,102 with respect to either the linear tapered conical resistors 91, 92or the cylindrical series resistor 95. A first order of compensation isafforded by conical resistors 91 and 92, as heretofore described. It hasbeen found unnecessary to compensate by tapering for series resistor 95in this pi-p'ad configuration. Should a higher order of compensation bedesirable, further tapering and matching by logarithmic or lineartapering means subtended over the conical resistors and/or the linearresistor may be used.

Fig. 9 is a cross-sectional view through a coaxial line termination unitutilized for absorbing all incident energy at a given characteristicimpedance. Termination unit 105 comprises cylindrical cup member 106within which is supported a single conical resistor 107, having an an-vnular conducting ring 108. The conical resistor 107 is constructed in amanner as hereinabove set forth, and comprises a low loss dielectriccore material 109 and a linear (or logarithmically) shaped surface 110upon which the resistive film is deposited. Coaxial cable hardwareconnection member 111 is arranged at the left end of termination 105 inthe manner of member 21 of T-pad of Fig. 1. Hardware member 111 isutilized to electrically and mechanically connect termination unit 105to the microwave circuit at a usual cable member.

A coacting metal cylinder 112 is arranged about conical resistor 107which in the illustrated embodiment is not tapered, but parallel to theaxis of unit 105. The left end of the shunt resistor 107 is connected tothe central conductor 113 of cable member 111, through connection clipor cup 114. A resilient shim 115 is arranged between cylinder 112 andthe outer surface of annular connection ring 108. A further springmember 116 is arranged between the conical resistor 107 and thecylindrical exterior member 106 of termination unit 105. It is to benoted that the assembly of termination unit 105 is mechanically andelectrically stable, suitable for all practical conditions ofutilization. Electrically, the conical taper of resistor 107 affordscompensation for frequencies from direct current up to 10 to 12 kmc.with low VSWR ratios. The heat dissipation is found to be adequate andwhere higher energy absorption is required, heat conduction fromtermination unit 105 may be arranged by well known means.

An important feature of the present invention is the utilization ofconical and bi-conical shunt resistor elements to provide a resistivecurrent path between the inner and the outer conductors of attenuationunits with minimum distortion of the microwave signal fields, and theprevention of standing waves. The basic angle of the resistive surfaceof such conical resistors corresponds to the rise of the linear surfaces58, 59 of unit 3 5, and of units 91 and 107; as well as the basic anglerise of the logarithmic surfaces 69, 70 of unit 65. Construction of theconical resistor surface at an angle to provide total internalreflection of the signal, results in very little energy being reflectedback toward the source of the signal, with all incident power beingdeveloped across the conical resistive face. Such conical angle is theoptimum, as will now be set forth.

Fig. 10 schematically indicates the basic angle 6 (axially) of theconical surface s with respect to the axis or direction of incidentmicrowave signal power. The conical surface s has a resistive film (r)upon it. The optimum angle 0 is such as to present surface s to thedirection P of incident power (axially) at the angle 0 of total internalreflection. Angles 6 and 9 are complementary, totalling The dielectricconstant e is that for the medium ahead of surface s, and is usually airwhere e =l. The dielectric constant e is that of the dielectric materialchosen for the conical unit.

The following relations set the optimum determination of 6 and 0 in thepractical design of conical shunt resistors for the attenuation units ofthe invention.

1 Sin 0,= i

V5 where e is air.

Thus

3 0,=sin i and (4) 62 (90 sin Where 0 (and 6 are proportioned inaccordance with these formulae, the Poynting vector in the cone C medium(e is parallel to the boundary surface (s) and consequently no averagepower crosses the boundary surface. All the power incident on surface sis developed across the resistive face (1').

The Formulae 1 to 4 are only to a first approximation, since thepresence of resistive face (1') affects the final value of 6 Thus theoptimum value for conical angle 9 is best determined empirically, beyondthe first approximation per Formula 4, with the resistivity of thematerial used for (1') included as a factor. Basically however, theangle 0 depends upon the choice of dielectric material (e since e isusually air. In practice, the angle 0 may lie between 20 and 70dependent upon the dielectric material for cone C as well as theresistive material for (1').

An optimum angle 0 for surface s in a practical attenuator results inthe incident microwave power P impinging on shunt resistor (r) of cone Cat the angle (9 of total internal reflection. However practical resultsare feasible for conical surfaces (s) at angles other than theprescribed optimum, in the invention attenuator pads. The order ofcompensation by use tapered shields or logarithmic conical resistivesurfaces, further minimizes reflections and VSWR.

While I have described and illustrated my invention with severalexemplary embodiments and explanations of its functional aspects,variations will be apparent to those skilled in the art, and I donot'intend to be limited except as set forth in the following claims.

What I claim is:

l. A high frequency tubular attenuation unit comprising a shunt resistorwith a solid core of homogeneous low-loss dielectric material having anouter conical surface, and a uniform film of resistance material acrossthe outer conical surface thereof, the conical core being supportedlongitudinally of the unit in coaxial relation therewith, and theresistance material thereon extending over a substantial axial pathalong the unit at a predetermined inclination thereto as a conicalenvelope tapered from the axial region of the unit transversely acrossthe tubular interior thereof to constitute a shunting impedancecontiguous with the homogeneous dielectric core therein forprogressively intercepting microwave signal energy passing through theunit with relatively low VSWIR over a wide frequency range.

2. A microwave attenuation unit as claimed in claim 1, in which theresistance surface of the conical core is inclined to the direction ofincident signal power at an angle between 20 and 70 such that the signalpower is substantially all developed across the resistive layer.

3. A high frequency tubular microwave attenuation unit comprising ashunt resistor composed of a solid conical homogeneous core of low-lossdielectric material with a uniform conical outer coating of resistancematerial and a transverse conductive outer ring at each end of theconical core in contact with the adjacent edge of the resistancematerial, said transverse rings being in individual planes perpendicularto the axis of the resistor and spaced across the unit forinterconnection of the resistor in the attenuation unit, the conicalcore being supported longitudinally of the unit in coaxial relationtherewith, and the resistance material thereon extending over asubstantial axial path along the unit at a predetermined inclinationthereto as a conical envelope tapered from the axial region of the unittransversely across the tubular interior thereof to constitute ashunting impedance contiguous with the homogeneous dielectric coretherein for progressively intercepting microwave signal energy passingthrough the unit with relatively low VSWR over a wide frequency range.

4. A microwave attenuation unit as claimed in claim 3, in which theresistance surface of the conical core is inclined to the direction ofincident signal power at such effective angle as to create totalinternal reflection, wherein the signal power is substantially alldeveloped across the resistive film.

5. A microwave attenuation unit as claimed in claim 3, further includingan integral annular ridge projecting outwardly from said core at theplane of the larger transverse conductive ring and integrated therewithfor firmly supporting the resistor within the unit across the ridge withits conical body held in substantially precise coaxial alignmenttherewithin with the smaller conductive ring extended across the axis ofthe unit for engaging with and supporting the coacting end of anadjacent resistance element of the unit.

6. A microwave attenuation unit as claimed in claim 5, further includinga cylindrical shield about said shunt resistor with one end abutting theannular ridge to support the resistor therewithin with its narrower endprojecting into the shield, the internal surface of said cylindricalshield being tapered substantially across its whole section that isopposite the shunt resistive coating, the taper inclining in thelongitudinal direction from the ridge end towards the smaller coatingdiameter to establish low VSWR characteristics therethrough over a widefrequency range.

7. A high frequency tubular attenuation unit comprising a shunt resistorcomposed of a solid conical dielectric core with a uniform film ofresistance material on the outer conical surface and an annularconductive ring at each end of the conical core in contact with theadjacent edge of the resistance material, said transverse rings being inindividual planes perpendicular to the axis of the resistor and spacedacross the unit for interconnection of the resistor in the attenuationunit, and conductive material arranged along the surface of a centrallongitudinal opening in the conical core to establish a substantiallyuniform conduction path through the interior of the dielectric corealong the axial region of the attenuation unit, the conductive ring atthe narrower resistor end being electrically connected to said interiorconduction path.

12 8. A microwave attenuation unit as claimed in claim 7, in which theresistance surface of the conical core is inclined to the direction ofincident signal power at an angle substantially the complement of 1BID-J: 0

(where a is the dielectric constant of the dielectric core), wherein thesignal power is substantially all developed across the resistive film.

9. An attenuation unit as claimed in claim 7, further including anannular ridge projecting integrally outwardly from said core at theplane ofthe larger transverse conductive ring and integrated therewithfor firmly supporting the shunt resistor within the unit across theridge with its conical body held in substantially precise coaxialalignment therewithin with the smaller conductive ring extended acrossthe axis of the unit for engaging with and supporting the coacting endof an adjacent resistance element of the unit.

10. An attenuation unit as claimed in claim 9, further including asecond conical shunt resistor the same as the first conical shuntresistor, said first and second shunt resistors being integral across acommon central annular ridge to constitute a symmetrical bi-directional.shunt resistor assembly supported in the unit across the central ridgewith both narrower core ends extending on opposite sides to engage withaxial series resistors of the unit, the interior conduction path beingcoextensive through the shunt resistor assembly and in electricalinterconnection with both smaller conductive rings thereof, and bothlarger conductive rings being in electrical connection across the commonridge.

11. An attenuation unit as claimed in claim 10, further including acylindrical shield about each of said shunt resistors, each shieldhaving an end abutting the common annular ridge on each sde for firmlysupportng the resistor assembly in the precise alignment and for itsengagement with the axial series resistors, said shields electricallyconnecting with the larger conductive rings at the ridge region.

12. An attenuation unit as claimed in claim 11, further including aseries resistor supported within each of said shields, one end of eachseries resistor being electrically engaged with and supported by theprojecting narrower end of the associated shunt resistor, and theinternal surface of each of said shields being tapered opposite theresistive portions of said shunt resistors, the shield taperingextending in the longitudinal direction from the ridge towards therespective smaller shunt-resistor diameters to establish low VSWRcharacteristics therethrough over a wide frequency range.

References Cited in the file of this patent UNITED STATES PATENTS2,262,134 Brown Nov. 11, 1941 2,471,732 Feenberg May 31, 1949 2,557,122Leiphart June 19, 1951 2,620,396 Johnson et al. Dec. 2, 1952 2,672,591Wallauschek Mar. 16, 1954 FOREIGN PATENTS 882,871 Germany July 13, 1953OTHER REFERENCES German application T7634 VIIIa/ 21a4, June 14, 1956.

